1. Field of the Invention
The present invention relates to a method for executing a control instruction in a multi-computer system, and is particularly concerned with a method for executing a computer control instruction in a manner suitable for preventing the system from executing a control instruction likely to cause a failure and thus for improving the reliability of the system.
2. Description of the Prior Art
Today, computers are widely used, and their reliability is a serious matter.
Due to such causes such as operators' careless pressing of keys and/or switched and, an inappropriate control instruction mistakes in programming input to a process in the middle of execution can lead to a computer's malfunction.
For example, the pressing of the power switch by simple operational mistake cuts off the power, likely to cause such damage as the loss of data being processed during execution and the destruction of a program in the middle of its execution.
To prevent such cutoff of the power by a careless pressing of the power switch, some conventional methods for controlling power OFF control instructions in a computer system do not permit turning off of the power switch for a stand-alone computer when the computer is performing a process in its CPU, as found in Japanese Patent Application Laid-Open No. 1-263811.
As for master-slave computer systems, such a method is explained on pages 46 to 49 of "HITAC L-70 System Overview" by Hitachi, Ltd. In this one, a master computer manages the power for a plurality of slave computers, and the power for the master is never cut off before all the managed power supplies for the slaves are cut off.
Apart from the power switch, a control signal such as a reset signal also causes a computer to stop a process in the middle of execution. Therefore, some conventional methods attempt to prevent a careless pressing of the reset switch, which issues a reset signal, by giving an elaborate location to the switch. In other conventional methods, systems ignore a reset signal itself if by checking they find out that the reset switch is pressed at inappropriate time.
Today, as data handled by computers increases in amount and importance, computers need to speed up their processing and improve their reliability. To meet such requirements, a multicomputer system has been developed, which is a distributed processing system for data processing operation shared by a plurality of computers. In this system, the distribution of loads helps to maintain the performance of each computer and also to speed up the entire processing by the whole system and further to handle the occurrence of a failure to one of the computers by using another computer as a backup to carry out the processing in place of the failing computer, an improvement of the reliability of the system.
In such a multi-computer system, since processing efficiency is increased by distributing the processing operation to computers according to the amount of data to be processed and thereby dividing loads at busiest time, a sudden cutoff of the power for computers connected to the system is likely to stop the processing that they have been working on, having received an apportioned processing workload from other computers, leading to data loss or system failure.
For a system composed of computers connected to each other, prevention of the cutoff of the power due to a careless pressing of the power switch is presented by such a prior art attempt as shown on pages 46 to 49 of "HITAC L-70 System Overview" by Hitachi, Ltd.
In this attempt, however, the power for slave computers is permitted to be cut off regardless of their relationship to the master computer. Therefore, for instance, even when a slave computer is functioning as a workstation and running a program on the master, the power for the slave can be cut off by mistakenly setting the power switch on the slave to OFF. For this reason, this method for managing the power switch has had no use in multi-computer systems.
In other words, applying conventional methods to multi-computer systems has the disadvantage that there is no operation for checking the status of other related computers being performed before cutting off the power which leads to the interruption of communication between computers and the imposition of a harmful influence on the consistency of resources.
Moreover, this conventional application has the disadvantage that similar problems are caused by the inappropriate input and execution of control instructions such as a reset instruction.